Provision to extension of react-table instance

[hitendramalviya]

First thing I would like give kudos to this community special mention for @KevinVandy, this product is really amazing & solving all modern edge cases and saving lots of time for the one who decided to use this package.

I got an idea to allow consumer to extend the table features, and by exposing a way to extend table instance. I tried extending though tableInstanceRef but that instance extension getting vanished every time TableRoot re-renders.

If I get chance I will open a PR. Thanks 👍